Trunking arrangement for automatic telephone systems



.Oct. 6, 1925 1,556,307

R. G. RICHARDSON TRUNKING ARRANGEMENT FOR AUTOMATIC TELEPHONE SYSTEMS Filed Oct. 11, 1921 5 Sheets-Sheet 1 Fig: i

I v nUEnIUr- Bulimia-g EBwhard'smn Oct. 6,1925. 1,556,307

R. G. RICHARDSON TRUNKI'NG ARRANGEMENT FOR AUTOMATIC TELEPHONE SYSTEMS Filed Oct. 11, 1921 5 Sheets-Sheet 2 HIT Oct. 6, 1925. 1,556,307

R. G. RICHARDSON TRUNKING ARRANGEMENT FOR AUTOMATIC TELEPHONE SYSTEMS Filed Oct. 11, 1921 5 Sheets-Sheet 3 Inusn Elf-- Hucfneg E. Hwha'rd s on Oct. 6,1925.

R. G. RICHARDSON TRUNKING ARRANGEMENT FOR AUTOMATIC TELEPHONE SYSTEMS Filed Oct. 11, 1921 5 Sheets-Sheet 4 j J1 J3 J! I Imamd'ua Efiwhard'sun' HIT g5 R. G. RICHARDSON TRUNKING ARRANGEMENT FOR AUTOMATIC TELEPHONE SYSTEMS Oct. 6, 1,556,307

Filed Oct. 11, 1921 5 Sheets-Sheet 5 UEHE-BIC}LEI?5UTL JAM WHTF A Patented Oct. 6, 1925.

RODNEY G. RICHARDSON, OF CHICAGO, COMPANY, OF CHICAGO, ILL

TES

ILLINOIS, ASSIGNORY 'ro AUTOMATIC n no'rn Io PATENT OFFICE INOIS, A CORPORATION OF ILLINOIS.

TRUNKING ARRANGEMENT FOR AUTOIVLl-EJIJIC TELEPHONE SYSTEMS.

Application filed October 11, 1921'.

T 0 all whom it may concern:

Be it known that I, RODNEY G son, a citizen of the United ca, and a resident of Chicago, and State of Illinois,

. RICHARD- States of Amen- Cook County, have invented certain new and useful Improvements in Trunking Arran ement for Automatic Telephone Systems, of which the following is a spec1fi cation. g

The present lnvention relates t 0 trunking arrangements for automatic telephone systems in general, but is concerned more particularly with trunking arrangements for such systems as employ non-numerical trunk selecting switches that I are seized by automatic selector switches; and the principal object, briefly stated, i

vision of new s the proand improved trunlnng arrangements whereby the trunk selecting movement of a non-numerical switch in a system of this type,

or the maximum number of trunks which such a switch will have to test to find an idle one, i ably reduced.

s consider- Another object is to provide an improved non-numerical switch using a plurality of relays to select an idle trunk instead of the usual step by step movement.-

' Still another object is to provide improved circuits for rendering all the trunksincoming to a particular secondary group busy while the trunks outgoing from are allin use.

that group A still further object isthe provision of new and improved circuit arrangements whereby the operation of a selector switch 1s improved.

These objects, together with are not specifically. pointed out others that now, will be more fully pointed out hereinafter,freference being had to the accompanying drawings comprising show by means of the usual circui Figs. 15, inclusive, which t diagrams sufficient of the apparatus employed in a system embodying the principles vention to enable the same In the drawings, placed together with Fig. 1 over with the corresponding of the in to be understood. Figs. '1: and 2, when Fig; 2 and lines at the ends of the sheets in alignment, represent a trunk} ing layout in which a major group of thirty i trunks shown on the right hand side of the two sheets is accessible through non-numerical switches to the major grou then each subgroup Serial No. 587,009.

trunks shown on'the left hand side of the two sheets. l

Figs.,3 and 4', whenarranged in order with the corresponding lines in alignment, represent sufiicient of the apparatus used in the trunking layout shown in Figs. 1

and 2 to enable the same to be understood. 7 Fig. 5 is a circuit diagram showing a group of non-numerical switches which are different from those shown in Fig. 3. These switches are of a type which may also be used in a layout such as is shown in Figs. 1 and 2, and for convenience it will be assumed that the system includes both types of switches. 1

Referring now particularly to Figs. 1 and 2, the trunking layout shown therein will be explained more in detail. It has heretofore been the practice, when the so called plunger type of non-numerical switches are used in similar lay0uts,to' divide the major group: of outgoing trunks, or the group which corresponds to the group shown on the right hand side into subgroups of I cordance with thenumber of trunksto which a plunger type switch has access. Similarly, when step by step rotary non-numerical divide the major group of outgoing trunks into subgroups of twenty five trunks each, in accordance with the number of trunks to which a switch of this type has access. This practice, so far as is known, has been followed invariably, regardless of the number of trunks in themajor group.- In the present case, however, the major group of outgoing trunks is preferably divided into ten subgroups, corresponding to the number of trunks to which a selector switch has access in any one level. As shown, there are thirty outgoing trunks in the major group and consequently there are three outgoing trunks in each subgroup. If the major group comprised forty outgoing trunks then each subgroup would comprise four trunks, or if the major group comprised only twenty trunks would comprise only two trunks. It will be seen now that as long as the major group of outgoing trunks is less than 100 there will not be as many as ten trunks per subgroup, which heretofore has been the minimum number, and with major groupsof a number much less than of the two drawings, ten trunks each, in acswitches. are used the practice has been to 100 the number of trunks in each subgroup is correspondingly less and consequently the switches which have access to these subgroups of trunks have a correspondingly smaller maximum number of trunks to test in order to find an idle one.

It will be seen from an examination of the drawings that there is a trunk line eX- tending from each selector group on the left to a non-numerical switch in each subgroup on the right. In the first-group of selector trunks, the group in the upper left hand corner of Fig. l, the first trunk leads to a non-numerical switch in the first subgroup, or the subgroup in the upper right hand corner of Fig.1. The second trunk in this selector group leads to a switch in the second subgroup, the tliird-trunkto a switch in the third subgroup and so on. The trunks from the second group of selectors extend to switches in the several sub groups in the same manner as those from the first selector group, except'that aslip is introduced andtrunk No. l in the second selector group leads to a switch inthe third subgroup, the second trunk -.to a switch in the fourth subgroup, and so on down to the last trunk in this group which leadsto a switch in the second subgroup. It will beobserred that this same slip is carried out all along down the line. This slip dist-ributes the traffic evenly amongst the various subgroups.

This revision of the trunking arrange ment, in decreasing the number of trunks in each subgroup,'not only decreases the time required for an idlevtrunk .to be selected, but when relay switches are .used'effects a great saving in equipment for the reason that the number of relays in such a switch is proportionate to the number of trunks to which the switch has'access.

Referring now particularly"to Fig, '3, the selector D shown therein is mechanic-ally of the usual vertical andvrotaiy type and accoirlingly has its bank contacts arranged in horizontal rows or levels. This switch 'is operated according to the well known two-wire system of control. The circuits of this switch are shown complete and will be described fully hereinafter.

The non-numerical switch E which is accessible to the selector D, 'does' not have the usual mechanical trunk selecting movement but has instead a plurality of relays which operate one after the other to select an idle trunk. The circuits of this switch also are shown complete and will be described fully hereinafter.

Each of the trunks to which the switch l lfltogether with. the other switches :in the same group, has access, is assumed to extend to a repeater, such as the repeaters F, F, and F Figure 4, and thence by way of a two conductor trunk line such as the trunk lines X, ,X", and X to another office in the system. These repeaters 'F, F, and F are all identical with each other and, with the exception of an added relay, are identical -w-iththet-wowirerepeaters that are now in common use.

The relay group: RGshown in the lower part of'Fi l is'used'forthe purpose of making all the trunks incoming to the group busy as soon as all the outgoing trunks are taken for use, or rather, assoon asthelast call arrives at the group. This operation will be explained-more fully hereinafter.

Of the switches H, H H and H Fig. 2,andof which. only H, H and H are shown in the detailed circuit drawing in Fig. 5, each is similar to the switch E Fig. 3, in that no mechanical step by step arrangement is employed to pick out an idle trunk line, but are different in that the trunks are preselected as will be explained more fully hereinafter.

The apparatus having been described more or less in general will now be described in connection with a detailed description of its operation. For this purpose, it will be assumed that a call comes through the exchange by way of the selectorD, Fig. 3. The selector D, it may be pointedout, is one of the selectors which has access to the second group of trunk lines on the left of Fig.2, counting from the bottom. We shall assume further that when the call comesin none of the switches EE*, inclusive, Fig. 2,.are in use and consequently all of the trunks outgoing from that subgroup are idle at this time. When the call comes in as assumed, the selector D is seized at the bank contacts 2, 3,,and 4, or their multiples, in which the trunk line comprising conductors 5, -6, and 7 terminates, and line relay 10 is energized over the calling subscribers loop in series with the impulse springs'o-fthe calling subscriber s callingdevice. Upon energizing, line relay l0 closes a circuit for release relay 11, including the resting contact of armature 30 of switching relay 14 and said armature, at-armature 15. Release relay l1, upon energizing, opens a point in the circuit of release magnet 19 and closes a point in the circuit of slow acting relay 12 at armature 1'7, and at armature 16 places ground on release trunk conductor 6,-thereby establishing a holding circuit for the switching relay of the switch which seized the selector D.

The calling subscriber now manipulates his calling device in accordance with the next digit in the desired number, thereby producin a corresponding number of in terruptions in the circuit .of line relay 10. As soon as line relay '10 falls back in response to the first interruption produced in its circuit, it closes at armature 15, the cirall cuit of-the slow acting relay 12. *The slow Vertical magnet 20 in multiple. Y Relays 11.

and 12, being slow acting, are both main tained energized throughout the series of interruptions. By the operation of the vertical magnet, which energizes each time its circuit-is closed following aninterruption in the circuit of the line relay 10, the switch shaft and wipers are raised step by step until the latter come to rest opposite the desired level of contacts, which will be sumed to be the level in which the trunk line comprising conductors 38-40, inclusive, terminates. As soon as the shaft of the switch is moved from itsnormal position the off-normal springs 26 close, thereby completing a circuit for stepping relay 13. Stepping relay 13, upon energizing, closes a locking circuit for itself at armature 25 in series with interrupter contacts 27, and at armature 24 prepares a circuit for rotary magnet 23. After the last interrupt-ion is produced in the circuit of line relay 10, armature comes to rest against its workingcontact thus leaving the circuit of slow acting relay '12 open. Upon deenergizing, relay 12 opens the initial circuit of stepping relay 13 at armature 22*and at armature 21 opens the circuit of vertical magnet and closes the circuit of rotary magnet 23. Rotary magnet 23, upon energizing, advances the wipers 32-34, inclusive into engagement with the first set of contacts in the level opposite which they are raised. Near'the end of its stroke rotary magnet 23 separates the interrupter contacts27, thereby opening the circuit of stepping relay 13. Stepping relay 13, upon deenergizing, opens another point in its circuit at armature 25 and at armature 24 opens the circuit of rotarymagnet 23. Rotary magnet 23 now deenergizes and closes the interrupter contacts 27 again.

From this point, the operation depends upon whether the trunkline which terminates in the first setof contacts is busy or idle. If it is busy, test wiper 33 is in engagement with a grounded contact andstepping relay 13 energizes again over a circuit which includes wiper '33 and armature 29 and its resting contact. Upon energizing, stepping relay 13 closes the previously described looking circuit for. itself at armature 25 and at armature 24 closes the circuit of rotary magnet 23. Rotary magnet 23, upon energizing, steps the wipers into engagement with the next set of contacts and agalnopens its 1nterrupter contacts 27. This alternate operation of the-rotary magnet and stepping relay continues until an idle :trunk. is found. In the present case,however, it has been assumed that the switches EE, inclusive, Fig. 2, are idle and consequently the trunk line terminating in bank contacts 35-37, in clusive, which is the first set of contacts in the level, is idle, when the call is made as. described. This being the case, there is no direct circuit closed for'stepping relay 13 when wiper 33 arrives upon test contacts 36 and" the switching relay 14 which has heretofore been short circuited immediately energizes over, the following path: From ground by way, of the working contact'of armature 16 and the said armature, switching relay 14, interrupter contacts 27, off normal contacts 26, and stepping relay 13 to battery. Stepping relay 13, however, being wound with comparatively few turns'is not able to energize .over this circuit on account of the high resistancevof switching relay 14. Switching relay 14, upon energizing, disconnects test wiper 33 from a point between its own winding and that of stepping relay 13 and connects it to the release trunk conductor 6 at armature 29; removes ground from the armature of line relay '10 at armature 30; and at arm'atures 28 and 31 disconnects the trunk conductors 5 and 7 from the upper and lower windings of line relay 10 and extends them by way of the working contacts of the said armatures, line wipers 32 and 34, bank contacts 35 and. 37, trunk conductors 38 and 40, armatures 58 and 61 and their resting contacts to the winding of line relay of switch E and ground, respectively. Line relay '50, upon energizing, places ground upon release trunk conductor 39 at armature, 57, thereby establishing a holding circuit for the selector D-and the switches backof it before the slow acting release relay, 11 has had time to deenergize. It will be noted that as soon as ground is'placed on" release trunk conductor 39 by the pulling up of switching relay 14 of. the,

switch D, the seized trunk is made busy immediatelyto theother switches having access to it. In this case, however, not only the seized trunk but also all the other trunks incoming to this group of switches is made busy through the common conductor 90 to which all the release trunk conductors are normally connected through the'resting contacts of armature '60, 60 etc. It is seen now that any other call which comes in atthis stage will not take a trunk leading to the group in which the switch E is located but will be'diverted to some other group.

' To continue with the operation of the switch E line relay 50, upon energizing, in addition to placing ground on release trunk conductor 39 as described,.closes at armature 56'the following circuit: From ground by ion particular function at this time.

way of armature 56 and its working contact, swltchlng relay 51, armature 69 and its resting contact, armature 74 and its resting contact, and the lower winding of the relay 5% to battery. This circuit, it will be noted, includes relay 51 and the lower winding of the relay 5% in series. As a further result of the operation of line relay 50, armature is connected to a point between the winding of relay 51 and the lower winding of relay 54:. Armature 55 is normally connected to ground through the resting contact of'armature 59 and said armature, armature 63 and its resting contact, armature 67 and its resting contact, and armature 7 2 and its resting contact. Switching relay 51 is accordingly short circuited and prevented from energizing. Relay 5 1, however, energizes; closesa locking circuit for itself at armature 73; disconnects ground from the point between its lower winding and that of switching relay 51 at armature 72 and connects up release trunk conductor 77 instead; disconnects its own lower winding and connects up the lower winding of relay at armature 7 1; andat armatures 71 and 75 connects up the upper and lower series of contacts and armatures to trunk conductors 7G and 78.

From this point the operation depends upon whether the trunk line comprising conductors 7678, inclusive, is busy or idle. If it is busy release trunk conductor 77 is grounded and switching relay 51 is still short circuited. With this condition existing relay 58 energizes; closes a locking circuit for its own upper windin and opens the locking circuit of switching relay 5% at armature 68; connects the point between its own lower winding and the winding of switching relay 51 to release trunk conductor 80 at armature 6T; disconnects its own lower winding and connects up the lower winding of relay 52 at armature 69; and 'at armatures 66 and 70 disconnects the trunk conductors 76 and 78 and connects up the tr'unk conductors 79 and 81. Relay '54 de- -energizes now in response to the opening of its locking circuit but does not perform any If the trunk line comprising conductors 79-81, inclusive, is busy at this time there is a ground potential on release trunk conductor 80 and relay 52 by action identical with that of relay 58, just described, transfers the connection to the trunk line comprising conductors 82-84, inclusive, and relay 53 deenergizes.

In the present case, however, we shall assume, as before stated, that the trunk line comprising conductors 7678, inclusive, is idle at this time and consequently there is no ground potential on release trunk conductor 77, and as soon as relay 54 has energized and transferred the circuits in the manner described, switching relay 51 energizes in series with'ther lower winding of relay 53. Relay 53, however,,does n0t energize in series with the switching relay 51 on account of the high resistance of the said switching-relay 51. "Switching relay 51, upon energizing, disconnects release trunk conductor77'from the point between its ownwinding-and the lower winding of relay 53at *armature 159 and connects it to ground through the 'working contact of armature .56 =and=said armature, thus causing the seized'trunlrto test busy immediately to the other-switches having access to it; disconnects release trunk conductor 39 from the common conductor 90 at armature and connects it by way of the previously traced series of'armatures and contacts to release trunk conductor 77; and at armatures 58 .and'61 disconnects trunk :conductors 38 and l0 from the winding of relay 50 and ground, respectively, and extends them by way of armatures 62 and and their resting contacts,'armatures :66 and and their resting contacts,;armatures 71- and and their working contacts, trunk conductors 76 and 78,;andthenormally closed contacts controlled .by armatures 110 and 111 to the :upper and lower windings of the double wound line relay: 102 of repeater F. Line relay 102 now-energizes over the calling 'subscriberls loop and at armature 113 opens a point in-theic-ircuit of slow acting relay 103 and closes-aicircuit for relays 104: and 105 .in':multiple. Relay 105, upon ener gizing, among other things which will be pointed out more fully hereinafter, disconnects relay lOl-fromthe-working contact of armature 113 and substitutes a circuit through armature 120 and its working contact. Relay 104, upon energizing, places ground upon release trunk conductor 77 at armature117. Thisoccurs before the slow acting line relay of the switch E has had time to deenergize and maintains a holding circuit for the switch'E and other switches behind it.

Returning now to the repeater F, as a further result of the energization of relay 10 1, it closes at armature 119 ,a circuit for the lowerwinding of electropolarized relay 106. This winding alone, however, consisting of a small numberof turns of a relativelyhigh resistance, is not able to effectively energize the relay. As a still further result of the energization of relay 10 1 it prepares a circuit for slow acting relay 103 at armature 118, and .at armature 116 completes a bridge across the conductors of the trunk line X as follows: From the lower conductor of trunk sline X by way of the working contact of armature 114 and said armature, right hand winding of impedance v107, normally closed contacts controlled by armature 112, .upper winding of electropolarizecl relay 106, resting contact of armature 115 and said armature, and the Working contact of armature 116 and said armature to the upper conductor of trunk line X. In response to the closure of this bridge, the line relay of the automatic switch on the other end of the trunk line X is energized and the switch is prepared for operation in the usual manner. Current is now flowing through both the upper and lower windings of electropolarizedrelay 106 but'the said relay is not energized at this time because the current flowing in either winding is opposite in direction to that flowing in'the other.

The calling subscriber now manipulates his calling device in accordance with the next digit in the desired number, thereby producing a corresponding number of interruptions in the circuit of line relay 102. Each time line relay 102 deenergizes in response to one of these interruptions it opens the circuit of the line relay of the automatic switch on the distant end of the trunk line. at armature 114, thereby causing the said switch to operate in the usual manner. 7 Slow acting relay 103 is maintained energized throughout the series of interruptions over a circuit which includes armature 113 and its resting contact and armature 118 and its working contact. Upon energizing, slow acting relay 103 completes the bridge across the conductors of the trunk line X by a moredirect path, which excludes the right hand winding of impedance 107 and the upper winding of electropolarized relay 106 at armature 1.15 and its working contact. This is the usual expedient made use of in order to obtain first class switch control. At the end of the series of interruptions, line relay 102 comes to rest in an energized condition and slow acting relay 103, upon deenergizing, restores the bridge across the conductors of the trunk line X to its previously traced path,

.The calling subscriber may now go ahead and dial the remaining digits of the desired number, the repeater F functioning for each digit in the manner described, When the connection is finally completed, and the called subscriber has been signalled and has responded by removing his receiver, the current flow in the trunk line X and consequently in the bridge across the-conductors of the said line in the repeater F is reversed by the final switch in the distant exchange. WVhen this occurs, the two windings of electropolarized relay 106 which heretofore have opposed each other now assist each other in energizing the relay. Upon energizing, relay 106 places a shunt around armature 115 and its resting contact at armature 124; places a shunt around armature 114 and its working contactat armature 126; and at armature 125 closes the circuit of m a s e y 1 -f pon sne ms,

ing trunk conductors 76 and 78 as regards Itheirconnetion with the upper and lower windings of line relay 102. This reversal of current flow is used in some cases to operate meters, coin boxes and the like but does not have anything in particular to do with the present invention, the circuits being shown in thls manner because of the fact that it is more or less a standard arrangement. The

calling and called subscribers may now converse with each other as desired, the talking circuits that are shown being outlined by heavy conductors.

When the conversation is completed the calling and called subscribers replace their receivers. W hen the receiver is replaced at the called substation the. current flow in the conductors of the trunk line Xis reversed back to normal whereuponelectropolarized relay 106 retracts its armatures and upon so doing. removes the shunt from around armature 115 and its resting contact at armature 124; removes the shunt from around armature 114 and its working contact at armature 126; and at armature 125 opens the circuit of reversing relay 101. Reversing relay 101, upon deenergizing, reverses the current flow in the calling subscribers loop back to normal. This operation, however, does not perform any useful function in the present case.

lVhen the receiver is replaced at the calling substation the circuit of line relay 102 of repeater F is opened and the said relay, upon deenergizing, opens the bridge across the conductors of the trunk line X at armature 114, whereupon, the automatic switches in-the distant exchange restore. themselves to normal position in the usual manner. As a further result of the deenergization of line relay 10 2,it opens the circuit of slow acting release relay 105 and closes the circuit of slow acting relay 103 at armature 115. Re-

lease relay 105, upon deenergizing, opens the circuit of relay 104 at armature 120. Relay 104, upon deenergizing, opens the circuit of slow acting relay 103 at armature 118 and at armature 117 removes ground from release trunk conductor 7 7 When ground is removed from release trunk conductor 77, re-

lay 54 and switching relay 51 of the switch E and switching relay 14' of the selector D, together with the switching relays of the various other switches which were engaged in the connection before the selector D I was reached, deenergize. The deenergization of the two relays mentioned in the switch E restores that switch to normal condition and connects up release trunk conductor 39 to common conductor 90 at armature 60.

In the selector D, switching relay 14, up-

now be explained in detail.

on deenergizing, completes a circuit for release magnet 19 as follows: From ground by way of resting contact of armature and said armature, armature 15 and its resting contact, armature 17 and its resting contact, cit-normal contacts 18, and re lease magnet 19 to battery. By the operaion 01 the release magnet, the shaft and wipers of the selector D are restored to normal position in the usual manner, the circuit of release magnet 19 being opened at oft-normal contacts 18 by the switch shaft when it reaches normal position. The other switches engaged in the connection before the selector D was reached are restored to normal position in the usual manner. The entire connection is now released and the apparatus is now ready for another call.

Referring now again to Figs. 3 and 1, the function of the relay group RG, Figure 4, together with its associated equipment will We shall assume that the group of trunks, to which the switch E and others has access, is idle and that a call comes in over one of the switches, the switch E for example. As soon as the trunkline comprising conductors 33, 39 and 10 leading to the switch E is seized by a selector such as the selector D, a ground potential is thrown upon release trunk conductor 39 by the selector in order to make the seized trunk busy immediately. This ground potential, as hereinbe'fore pointed out, extends by way of the common conduc 'tor 90 to the release trunks of all the switches, in that group. This ground potential extenns also by way of conductor 90 to relay 130 of therelay group HG, Fig. 1. Relay130, upon energizing, connects the common conductor 99 with conductor 14-2 at armature 13 1. This operation, however, does not perform any particular function at this time. As soon as the various relays of the switch E have had time to function, release trunk conductor 39 is disconnected from common conductor 90 at armature of switching relay 51, whereupon relay 130 of relay group RG deenergizes and dis connects the common conductor 90 from conductor 1 12.

As a further result of the energization of switching relay 51 otthe switch E the trunk line extending to the repeater F is seized and the repeater is made ready for operation in a manner hereinbefore expla ned. As soon as relay 105 of repeater F' is energized, the first one of a chain of contacts in the circuit of relay 132 of relay group 12G. is closed at armature 121, and at armature 122 the resistance 123 is disconnected from conductor 139 and ground is placed on the latter, thereby closing a circuit for relay 131 of relay group RG by way of armatu'res'133'and 137. A circuit is closed at this tim for relay 133 of relay group Rt} as follows: F romground by way of conductor 139, armatures 122 and 122 in multiple and their resting contacts, re-

sistances 123 and 123 conductor 1 11, and Y relay 133 to battery. Relays 131 and 133 energize simultaneously and the latter, upon energizing, opens the circuit of the former at ari'nature 138, whereupon, the said relay 131 deenergizes. It is seen now that with one ofthe repeaters of the group comprisingrepeaters F, F" and F busy, ground is placed on conductor 139 to close the circuit of relay 131' but relay 133 is energized and has the circuit of the former relay open.

We shall assume that another call comes in and that one of the non-numerical switches in the group in which the switch E5 is located seizes one of the remaining repeaters, the repeater F for example. As soon as the non-numerical switch is seized, ground is placed on the common conductor 90in a manner describedbefore but does not produce any result in the relay group other than merely operating relay 130. Assoon as-thenon-numerical switch has had time to function and seize the repeater F ground is removed tromthe common conductor 90 and relay 130 deenergizes. Relay 105 of the repeater F upon energizing, closes another point in the circuit of relay 132 at armature 121 removes the resistance 123 from the circuit of relay 133 and places a multiple ground on conduct-or. 139 at armature 122 Relay 133,.which is marginally adjusted, deenergizes at this time since its circuit is closed through only the resistance 123 and completes the circuit of relay 131 at-arma-ture138.- Relay 131,.upon. energizing, places ground on conductor 14-2 at armature 135. This is done in'preparation so as to place ground on the common conductor 90 at relay130" assoon as the sole remaining outlet from this group is taken for use.

Assuming now that another non-numerical switch in this group is seized, ground is thrown upon common conductor 90 in the usual manner, whereupon relay 130 of the relay group RG energizes and connects the common conductor 90. with conductor 14-2 which is grounded at armature 135 of relay 131. As soon as the non-numerical switch has functioned, its release trunk is disconnected from the common conductor 90 and the repeater F is seized in the usual manner. All the outlets from this group of switches are in use and if another call were permitted to come into this group at this time it would be lost for the lack of an outlet. lo call cancome into this group at this time, however, for the reason that the com mon conductor 90 is maintained grounded by relay 130 of relay group RG and, consequently, the calls are shifted to such other groups of non-numerical switches as have idle trunks leading out from them. As soon as relay 105' of the repeater F has energized in response to seizure of this repeater, it completely opens the circuit of relay 133 at armature 122. This, however, does not produce any effect in the relay group RG as relay 133 is already deenergized. As a further result of'the energization'of relay 105' it completes the circuit of relay 132 at armature 121. Relay 132, upon energizing, opens the circuit of relay 131 at armature 137 and at armature 136 places a multiple ground on conductor 142 and consequently on common conductor 90. Relay 131, upon deenergizing, disconnects conductor 142 fromvground at armature 135' but the said conductor is still groundedat armature 136.

We shall assume for the sake of illustration that the subscriber who is using the switch E and the repeater F finishes his conversation and replaces his receiver. Vith the assumption as noted, the connection is released in a manner hereinbefore described and release trunk conductor 39 of the switch E is connected to the common conductor again. As soon as relay of repeater F deenergizes, a circuit is closed for relay 133 of the relay group RG in series with resistance 123 but owing to its marginal adjustment, relay 133 is not energized at this time. As a further result of thedeenergization of relay 105, it opens the circuit of relay 132 at armature 121. Relay 132, upon deenergizing, removes ground from conductor 142 and consequently from the common conductor 90 at armature 136 and at armature 137 closes the circuit of relay 131. Relays 130 and 131 are so adjusted that the armature of relay 130 falls back I slightly quicker than the armature of relay 131 is attracted and consequently relay 130 deenergizes at this time and disconnects the common conductor 90 from conductor 142 at armature 134 before relay 131 has had time to energize. The circuit conditions of the relay group RGv are nowwthe same as they were just before the repeater F was seized, that is, conductor 142 is grounded and the circuits are in readiness for placing ground on the common conductor 90 as soon as the last outlet from this group is taken for use.

the circuitof relay 132. at armature/121 and at armature 122 places resistance 123 in multiple with resistance 123" whereupon the marginally adjusted relay 133 energizes and opens the circuit ofrelay 131 at' armature 138. Relay 131 now deenergizes and removes ground from conductor 142 at armature 135. r

It will be seen now that each time a switch in this group is taken for use the common conductor 90, which has the release trunk conductors of all the idle inlets connected with it, is connected with the conductor 142 of'the relay group RG, and if there is only one remaining outlet from the group there is a ground potential on-conductor 142, and relay 130 will become locked. Consequently, when the switch cuts through and the last outlet is taken for use, conductor 90 is not cleared but remains connected with the grounded conductor 142, thereby busying all the inlets into this group. It is seen further that the seizure of the last outlet is accompaniedv by a transfer of the ground connection on conductor 142 so as to permit the unlocking of relay 130 as soon as an outlet again becomes available. The advantageof the arrangement described lies in the fact 7 that when there is only oneavailable outlet from a group, the busying of the inlets to such group will occur simultaneously vwith the seizure of the neXt switch, and all possibility of a call being extended to a secondary, switch having no idle outlets is eliminated.

It is to be understood that this same arrangement of making incoming trunks busy may be used in a situation in which there are more outlets than are shown in the present drawings. When other outlets are added, the repeaters associated therewith,

will have their make busy contacts connected in the same manner as those of the in-. termed1ate repeater F and these other repeaters may be inserted between either the repeaters F and F or the repeaters F and F I It will be convenient to explain at this time that the switch E Figure 3, may be given access to as many outgoing trunks as desired by adding more trunk selecting relays and that, without adding any relays or other equipment, an additional trunk can beconnected to the resting contacts of armatures 71, 72, and 75 of relay 54. In that case, of course, the ground which is shown connected to the resting contacts of arma-- ture 72 is removed. With this extra trunk, if there is no ground on the resting contact of armature 7 2 of relay 54 when the switch E is seized, itbeing assumed of course that the trunk in question is idle, then the'energization of line relay 50 is followed immediately by the energization of switching relay 51 ,which switchesthe connection through to the normally connected trunkline. In case still-more trunks are desired they can be had,'as before stated, by adding one relay per trunk together with its associated armatures and contacts. Each of these added relays may have a spring assembly identical with that of relay 53 and may be inserted either between relays 52 and 53 or between relays 58 and 5 1. This increase in trunk capacity may be carried out as far as is desired, or until the cost. of the switch equals or surpasses the cost of a switch having a mechanical step by step trunk selecting movement.

It will be stated at this point that switches similar to the switch E Fig. 3, are assumed to be used in the entire layout shown in Figs. 1 and 2 with the exception of the group comprising switches HH inclusive, which are of a different type. This assumption however is made solely for convenience and to avoid the necessity of duplicating the layout, it being understood that in practice switches of one type or the other will ordinarily be used exclusively in any given situation.

Referring now particularly to Fig. 5, which is a circuit drawing of the switches H, H, and H of the group comprising switches HH inclusive, Fig. 2, the switches H and H are omitted, their circuits being identical with the circuits of switch H. For the purpose of illustration it will be assumed that all the switches in the group comprising switches HH in elusive, are idle at this time and that the switch H is seized from one of the selectors which has access to the trunk line comprising conductors 201-203, inclusive. Vith the assumption as noted, a ground potential is placed upon the release trunk conductor 202 immediately by the selector so as to make the seized trunk busy to the other switches having access to it. This ground potential extends by way of armature 216 and its resting contact, armature and its resting contact, and armature 225 and its resting contact, to common conductor 292 to which all the release trunk conductors of the incoming trunks are normally connected. This makes all the incoming trunks-associated with this group temporarily busy and prevents two calls from being received in this group simultaneously, the other calls being shifted to other groups. This ground potential extends also through armature 218 and its resting contact, armature 224 and its resting contact, and armature 226 and its resting contact to conductor 2% to which the trunk selecting relays 210. 211, and 212 are normally connected. Similarly this ground potential extends by way of a similar series of armatures and contacts in the switch H to conductor 295 to which all the relays of this switch-are normally connected. The same thing istrue for each. of the other switches in the group. The other terminal of the relays 212, 2 16, and 249 are normally connected to common conductor 289, and relay 211 together with the similarly located relays in the other switches is connected to the common conductor 287. Neither common conductor 287 nor common conductor 289 has a battery potential on it at this time and consequently none of the relays mentioned energize. Common conductor 285, to which relay 210 and the similarly located relays in the other switches are normally connected, is connected to battery at this time through the resting contact of armature 221 and said armature, resting contact of armature 232 and said armature, similar contacts and armatures on the switches H and H and the resting contact of armature 252 and said armature. That being the case, the first relay in each switchin this group starts to energize as soon as ground is placed upon release trunk conductor 202 as previously described. Relay 210, upon energizing, disconnects release trunk conductor 202 from common conductor 292 at armature 216, thereby making the remaining switches in this group accessible and allowing all the other relays that started to pull up, responsive to the placing of a ground potential on conductor 292, to fall back. As a further result of the energization of relay 210 it looks itself to release trunk conductor 202 and disconnects itself from conductor 294: at armature 219; locks itself to battery and disconnects itself from common conductor 285 at armature 220; places ground on conductor 290 at armature 217 disconnects re lease tr'unk conductor 202 from conductor 294. at armature 218; removes battery from conductor 285 and places it upon conductor 286 at armature 221; connects release trunk conductor 202 with release trunk conductor 276 of the seized trunk at armature 216; and at armatures 215 and 222 connects conductors 201 and 203 with conductors 275 and 277 of the seized trunk.- The line and release relays of the automatic repeater to which the trunk line comprising conductors 275277, inclusive, extends, immediately energize and prepare the switch for operation in the usual manner, and the latter, upon energizing, places ground upon release trunk conductor 276 thereby establishing the usual holding circuit.

The calling subscriber may now go ahead and dial the remaining digit of the desired number in the usual manner..

If we assume that another call comes in at this time and that the switch H is the one seized, a ground potential is sent forward over the release trunk conductor 205, through the series of contacts shown, to common conductor 292 and thence to the release conductors of all the idle switches in this group. The battery potential which was contact to conductor 287; -Accordingly 'r'elay 245 and the corresponding relays inthe switches H H ,'inclusive start to energize as soon as the ground potential is thrown upon release trunk conductor 205. There is no battery potential on common conductors 285 and 289, however, and consequently the associated relays do not energize. 1 Relay 245, upon energizing, disconnects release trunk conductor 205 from common conductor 292' at armature 234, whereupon, all the other relays which started to energize at this time deenergize'and the remaining idle switches are made accessiblegagain. As a further result oftheenergizatio'n of relay 245 it locks itself to release trunk conductor 205 and disconnects itself from conductor 295 atarmaturel-237 locks itself to battery and disconnects itselffrom conductor 287 at armature/238; extends therground poten tial which-is on conductor 290 to conductor 291 at armature 235; disconnects release trunk conductor 205 from conductor .295 at armature 236; disconnects'battery from conductor 287 and extends it to conductor 288 at. armature 239; connects release trunk conductor 205 with release trunk conductor 279 of the seized trunk at armature 234.; and at armatures 233 and 240 connects conductors 2 04 and 206 with trunkvcon'ductors 278 and 280 of the seized-trunk. The line'ia ndlre lease relays of the switch to which the trunk line comprising conductors 278-280, inclusive, extends now energize in the usual manner and the latter places ground on release trunk conductor 279, thereby establishing the usual-holding circuit. v V

- Assuming'now thatanother call comes in to this'group and'that it takes the switch H, the ground potential which is placed upon release trunk conductor 208 is extended forward to common conductor 292 and thence to the release trunksconductors of theswitchesin-this group which are idle at this time. The battery potential which wasplaced upon conductor 288 by the energization' of relay 245 ofithe switch H extends by way of armature 262 and its resting. contact, corresponding armatures and contacts in the switches H and H armature 243 and its resting contact, and armature 227 and its resting contact to conductor 289. I As a result relay 249 and the'corresponding relays in switches H and H comlnence to energize as soon as the ground potentialis placed upon release trunk conductor 208. In-this case there is no battery potential upon conductors 285 and 287 and consequent-ly'neither relay 247 nor 248 of the switch H nor the corresponding relays inf'the other switches'in this group which are idle'at this time can energ'ize Relay 249, upon energizing, removesthe ground potential from common conductor 292 at armature. 257

whereupon the corresponding -rel'ays in the switches H and H Fig.'2,' fall back. As

a further i result of/the, energization ,of relay 249 itlocks itself to release trunk conductor 208 :and' disconnects itself from conductor 295 at armature 260; locks'itself to battery and disconnects itself from conductor 289 atarmature 261; extends the ground po tential wh'ichis on conductor 291 to comrnon' conductor 292 at-farmature 258 disconnects release trunk conductor 208 from conductor 295 at armaturei '259;x disconnects battery from conductor 289 at armature 262 con-* nects releaseitrunk; conductor 208 with 'release trunk conductor 282 of the seized trunk at armature 257; and at' armatur'es 256'and 263 connects up conductors 207 and 209 with conductors 281 and 283, respectively, of "the trunk 'line comprising conductors 28l283,

inclusive." The line and release-relays of the switch to which the seized trunk extends now-energize in the usual manner and the latter: places 7 ground. on. release trunk con ductor; 282 thereby establishing the usual holding circuit. All the outlets from this "subgroup of switches'are now. in use and if a'call were permitted to come into this group at this timeit would belostvfor thelack of'an outlet. Calls are prevented from coming into this group at this time by theground potential which is extended forward by way of" conductors 290 and 291 tocommon con ductor 292 to which the release-trunk conductors of the idle switches H andH are connected.

In case the switch H becomes idle the battery. potential will be replaced upon common conductor 285 thus making the first outgoing trunk available, and at the same time ground is removed from conductor 290 and consequently from common conductor 292 thus making the idle switchesin this group available again. On the other hand if .we consider that thevswitch H becomes idle while the switch This yet busy the battery potential will be replaced on trunks which are :used the most are subjected to themost wear and may have .to' be replaced. often while .theothersaare practically unused except during the peak of the load which occurs periodically. This seeming difiiculty'can-be obviated, if desired, in a manner \VhlChWilliIlOW be pointed out. Working contacts =may be placed on armatures:227, 243, corresponding armatures on switches II and IF, and armature 262 to which arcommo-n conductor Vmay be wired. This common.conductor-may be extended to armature 252 of relay 2 H. 'This'makes the battery transfer circuit a complete loop. With this loop -,wired,as described the battery COIlIlQCtlQlL'DlZLY be removed from armature 25.2 and placed on' either armature 255 or armature 262, thus making :the associated trunk line first choice. Any suitable method 01' .lne'ans for shifting the battery potential from *onearmature to the other may be employed, for example, it may be shifted periodically by an attendant by means oftheswitchS, or it may beshifted upon the extension of ,eachcall by amaster switch arrangement.

It is to'be understood that in case ,more outgoing ztrunks :are desired they may be had by addingone relay to each switch together with the proper contacts and associated conductors. These :relays may be insertedbetween therow comprising relays 211, 245, etc. and the row comprising relays 212, 246, etc., or between the first named row. and the row comprising :relays 21.0, 244,-etc. It is to ,be'understood also that more switches -may be added in a group and that each-added switch willibe similar to the switch II" and maybe inserted anywhere along-the line between the switches H and H.

l/Vhile'the explanation mightzbe still .further amplified :it is thought that the foregoing is su'tficient to enable the invention to be thoroughly understood'by those who are skilled in the art and to whom this specificationvis directed.

The various features of the invention havingbeen thus described and ascertained will nowbe pointed out in the appended claims.

What is claimed is:

.1. In a telephone trunking system, a trunking section comprising a plurality of similar groups of inlets and a group of outlets divided into a pluralityof subgroups, the number of outlets in eachsubgroup being always equal to the total numberof outlets divided by the number of inlets in a group, irrespective of the total number of such outlets, and switching means for connecting inlets which are taken for use with idle outlets.

2. In a telephone trunking system, a trunking section comprising a plurality of similar groups of inlets and a group of outand switching means ;.for.connecting inlets.

which arez'taken for use with idle outlets.

In a telephone system, a secondary trunking section, step by step selector.

switches, .trunks incoming to said section from said selector switches, trunks outgoing from said section, each of saidtrun-ks comprising 'two talking conductors, .and switching means in said section consisting solely or relays for connecting incoming trunks which vare taken ;for :use with idle outgoing trunks. Y

34:. In a telephone system, a secondary trunking zsection, step Ebfy :step selector switches, trunks incoming to saidasection from said selector switches, :tru'nks outgo: ing from said section, :each of said trunks comprising two talking conductors, a plurality of switching devicesconsis'ting solely of relays, each incoming .trunk being-termina-ted in oneot' saiddevices, and means for energizing a relay in .the associated switching device when an incomingatrunk is taken for use to connect such trunk with an idle outgoing trunk.

5. In atelephone system,a line terminating in an. automatic switch, said switchcomprising a series of connectingarelays, aigroup of-trunk lines accessiblezto. said. switch, there being one trunk line for each connecting relay, means responsive to .a call .on .said

first 'line for energizing and deenergizing a plurality ofsaid relays in a'definitel order to connect said first line with successive trtmk lines, and means for preventing the energization o'flany more ofsaid relays as soonasa relay is energized which is associated with anidle trunk line. f

6. In a telephone system, means for establishing a talking connectionincluding an automatic trunk selecting switch, aline :re-

lay and a plurality of trunk connecting relays included in said switch, and circuits controlled by said iline relay upon a single energization thereof for energizing a plurality of said connecting relays successively to test for and select an idle trunk.

7. Ina telephone system, means including an automatictrunk selecting switch for connecting calling and called lines, said switch having access to a plurality of trunks and being provided with as many-trunk connecting relays as there are trunks, a line relay in said switch, and circuits controlled by said line relay responsive to a call for energizi1 1g said connecting relays ina definite order until an idle trunk line is selected.

8. In a telephone system, means including an automatic trunk selecting switch for connecting calling and called lines, said'switch having access to a plurality of trunks and being provided with asmany trunk connect ing relays as there are trunks; a line relay in said switch, circuits controlled byjsaid line relay 1 responsive to a call' for energizing saidQconne'cting relays in a definite order until an idle trunk line is selected,'{and a circuit independent of said line relay for maintaining the last energized connecting relay in operated position.

9. In a telephone'system, means including an automatic trunk selecting switch for connecting calling and called lines, said switch having access to'a plurality of trunks and being provided with as many trunk connecting relays as there are trunks, aline relay in said switch, a circuit completed by the line relay for energizing the first connecting relay, a circuit completed by each connecting relay for energizing the, next connecting relay, and testing: means for preventing [the energization of any more connecting relays assoon as a connecting relay corresponding to an idle trunk line is'energized. V I 10. I n a telephone system, means including an automatic trunk selecting switch for connecting calling and called lines,"said switch comprising a series of trunk connecting relays and a cooperating switching relay, a circuit for energizing a connecting re lay, and. a circuit for energizing the switching relay in series with the next connecting relay.'. 11. Ina telephone system, an automatic trunk selecting switch-for usein connecting 'callingfand called lines, said switch'comprising a switching'relay and a seriesof trunk connecting relays, fmeans for closing a'circuitincluding the switching relay and the first connecting relay, trunktesting means adapted to short circuit theswitching relay and cause the energization of the said first connecting relay, and switching means for substituting the second connecting relay in said circuit in place of the first connecting relay;

12. In a telephone system, an automatic trunk selecting switch for use in connecting calling and called lines, said switch comprising a switching relay and a series' of trunk connecting relays, atrunk -testing circuit and cooperating circuit transferring means for energizing a plurality of said connecting relays in' order until a relay is energized which is associated with an idle trunk line, and a circuit for said switching relay constituting a branch of said testing icircuit; I

13. In a telephone system, an automatic trunk selecting switch for use in connecting calling and called l1nes,' sa1d sw tch comprising a switching relay and a series-of trunk connecting relays, a circuit for said switching relay, circuit transferring means 'for-alteringsaid circuit to include successive connectingjrelays, and a T'testing. conductor connected to said cir'cu'it for short circuitin'g said switching relay and-Iorenergizing the'iconnecting relays. 1 i

, 14. In a telephonetrunking-system, a plurality of incoming trunks each terminating in a switching mechanism, a lesser plurality of outgoing trunks'accessible in common to all said mechanisms, and means effective whenever there is only one idle" outgoing trunk for making all remaining incoming trunks busy when the next incoming trunk is taken for use.

7 15.- In 'a telephone trunking system, apluu rality of incoming-trunks each terminating in a switching mechanism, a lesser plurality of outgoing trunks accessible in common to all said:mechanisms, and means effective in case there is; only one outgoing trunk available when an incoming trunk is taken for usefor simultaneously making such incomingvtrunk and the-remaining idle incoming trunks busy I i I- V 5 16, a telephone trunking systen'n'a plu-, rality'l of incoming trunks each terminating in a switching mechanism, a lesser plurality of outgoing trunks accessible in common to all said mechanisms,1means effective in case there is only one outgoing, trunk available when an incoming trunk is taken for use for simultaneously making such incoming trunk and the remaining idle incoming trunks busy, and means for maintaining such other incoming trunks busy until an outgoing I rality of incoming trunkseach terminating.

in a switching mechanism, :a lesser plurality of outgoing trunks accessible in common 1to all "saidf mechanisms, means. e'flective in case there is only one outgoing trunk available when an incoming trunk is taken for use {for simultaneously making 1 such incomingtrunk and the remaining idle. incoming trunks busy, and means for removing the busy condition on such remaining incoming trunks when any outgoing trunk becomesidle. I J

18. In atelephone trunking system, a plurality of incoming trunks each terminating in a switching mechanism, a lesser plurality of outgoing trunks accessible in. common to all said mechanisms, and means for making all idle incoming trunks busy whenever any incoming trunk is taken for use.

19. In a telephone trunking system, a plurality of incoming trunks each terminating in a switching mechanism, a lesser plurality of outgoing trunks accessible in commechanism associated with the incoming trunk in use is operated to select an idle outgoing trunk.

20. In a telephone trunking system, a plurality of incoming trunks each terminating in a switching mechanism, a lesser plurality of outgoing trunks accessible in common to all said mechanisms, means for making all idle incoming trunks busy when an' incoming trunk is taken for use, means for removing the busy condition on the idle incoming trunks as soon as the switching mechanism associated with the incoming trunk in use isoperated to select an idle outgoing trunk, and means for preventing such removalot the busy condition'in case no more outgoing trunks are idle.

21. In a telephone trunking system, a plurality of incoming trunks each terminating in a switching mechanism, a lesser plurality of outgoing-trunks accessible in common to all said mechanisms, and release trunk conductors for said incoming trunks all normally connected together over paths excluding any magnet windings of said switching mechanisms.

22. In a telephone trunking system, a plurality of incoming trunks each terminating in a switching mechanism, a lesser plurality of outgoing'trunks accessible in common to all said mechanisms, a release trunk conductor included in each incoming trunk, and means effective when an incoming trunk is taken for use for simultaneously grounding the release trunk conductorassociated with such incoming trunk and the release trunk conductors which are associated with all the other idle incoming trunks.

23. In a telephone trunking system, a plurality of incoming trunks each terminating in a switching mechanism, a lesser plurality of outgoing trunks accessible in common-to all said mechanisms, and means for busying "the excess incoming trunks when the outgoing trunks are all in use, said means in cluding'two relays, with means for energiz ing one relay when all but one of said outgoing trunks are busy and means for energizing the other relay when all of said outgoing trunks are busy.

In a telephone trunking system, a plurality of incoming trunks each terminating in a switching mechanism, a lesser plurality of outgoing trunks accessible in common to all said mechanisms, a release trunk conductor associated with each incoming trunk, means for grounding all the release trunk conductors associated with idle incoming trunks at one point when an incoming trunk is taken for use, means for grounding said release trunk conductors at another point while the switching mechanism associated with the incoming trunk in use is being operated, and means for grounding said release trunk conductors at still another point in case there isonlyone idle outgoing trunk available for selection by said switching mechanism.

25. In atelephonetrunkingsystem, a pluralityof incoming trunks, alesserplurality of outgoing trunks, switching mechanisms whereby any incoming trunk may be connected with'any outgoing trunk, a release trunk conductor included in each incoming trunk, and means effective when an incomingtrunk is taken for use for simultaneouslygrounding the release trunk conductor associated with such incoming trunk and the release trunk conductors which are associated with all the other idle incoming trunks. I 26. In a telephone trunking system, a plurality of incoming trunks, a lesser plurality of outgoing trunks, switching mechanisms whereby a connection may be effected between any incoming trunk and any outgoing trunk, and means for busying the vexcess incoming trunks when the outgoing trunks are all in use, said means including two relays, with means for energizing one relay when all but one of said outgoing trunks are busy, and means for energizing the other relay when all of said outgoing trunks are busy.

27. In a telephone trunking system, a trunking unit comprising a plurality of in coming trunks, a lesser plurality of outgoing trunks, and switching mechanisms whereby a connection may be effected between any incoming trunk and any outgoing trunk, means for making all idle incoming trunks'busy responsiveto the taking for, use of a single incoming trunk, means responsive to'the completion of a connection between said single incoming trunk and an outgoing trunk for removing the said busy condition from the remaining idle incoming trunks, and means commonto the trunking unit for placing a further busy condition on the remaining idle incoming trunks prior to the said removal ofthe first busy condition in caseonly one outgoing trunk is idle when the said'single incoming trunk is taken for use.

28. In a telephone trunking system, a

plurality of incoming trunks, a lesser pluality of outgoing trunks, switching mechanisms whereby a connection may be effected between any incoming trunk and any outgoing trunk, means for operating one 0]": said switching mechanisms when an incoming trunk is taken for use to eli'ect-a connection between such incoming trunk and an idle outgoing trunk, and means responsive to the takingof such incoming trunk for use for making other incoming trunks busy before the last named mechanism .is operated.

29. In a telephone trunking system,a.plurality of incoming trunks, a lesser plurality of outgoing trunks, switching mechanisms taking of such incoming trunk for use and whereby a connection may be effected beindependent of the said operation of the last 10 tween any incoming trunk and any outgoing named switching mechanism for making trunk, means for operating one of said other incoming trunks busy.

5 switching mechanisms when an incoming In witness whereof, I hereunto subscribe trunk is taken for use to efiect a connection my name this 28th day of September, A. D.,

between such incoming trunk and an idle 1921. outgoing trunk, and means responsive to the RODNEY G. RICHARDSON. 

